Conditions affecting AtT-20 cell glucocorticoid uptake

Glucocorticoid uptake by intact AtT-20/D-1 cells was studied to determine if the extent of uptake was enhanced or retarded by binding components in serum. The results demonstrate that the uptake of corticosterone, which binds to transcortin, was reduced by addition of serum while uptake of triamcinolone acetonide, which is not bound by transcortin, was unaffected. Neither heat denatured serum nor bovine serum albumin affected corticosterone uptake, further emphasizing the specificity of the inhibition. The presence of serum also affected the apparent binding specificity, since steroids able to bind to transcortin became less effective competitors when serum was present in the incubation medium. In the absence of serum, the specificity of glucocorticoid uptake was qualitatively similar to that of the isolated cytosol receptor. These results emphasize that the selective inhibitory effect of serum transcortin on whole cell uptake of certain steroids should be considered when assaying steroid potency in intact cellular systems.